they are plumped to reciculate though the canister (the larger pump). a trickle is all thats needed from the reactor you can plumb it off the recerulating loop.

you could do this by installing a bulk head into the filter.. but to be perfectly honest with you using a eheim is going to be cost proibitive to this project.. Coralife has a calcium reator on the market now that would make this project too expensive comparitivly.

You can use any water tight container and pump to build a calcium reactor so thinking this though is not a total loss.